The invention is based on a priority application EP 00 440 228.5 which is hereby incorporated by reference.
The invention relates to a method of generating a clock signal (clk) from an optical input signal (1) that is divided up optically into two channels, one for data extraction to generate an electrical data signal (11) and one for clock extraction to generate an electrical first clock signal (21) and also a digital optical receiver having an optical channel for data extraction and an optical channel for clock extraction, both channels each comprising means for converting the optical received signal into an electrical signal.
During the transmission of optical digital signals, various dispersion effects occur that are caused by the optical waveguides of the optical transmission system. In addition, the useful power decreases with increasing transmission distance with respect to the noise power because of the attenuation properties of the optical waveguide. At high bit rate (gigabit range), in particular, these effects result in falsification of the transmitted data signals. Optical receivers that are used in such a transmission system are confronted with the task of evaluating even falsified signals and, in this connection, to permit only low bit error rates. For this purpose, the received optical signal is converted into an electrical signal, sampled with the signal clock and then conveyed to a decision circuit in which a digital decision is made by a comparison with a threshold. A substantial problem in this connection is the recovery of the signal clock from the transmitted signal. Existing receivers frequently solve the problem in that the received optical signal is first converted into an electrical signal by means of a photodiode and is fed to the said decision circuit. The said electrical signal is cut off by means of a so-called splitter and fed to a clock recovery circuit. The clock or sampling signal extracted by the clock recovery circuit is fed to the said decision device, where the data signal is then sampled. It has been found, however, that, in particular at bit rates above 5 gigabit, phase and amplitude interferences in the electrical data signals occur because of increasing electromagnetic radiation from the highly integrated circuits of the receiver. These errors result in an increased bit error rate.
EP 0 817 408 A1 discloses a receiver for an optical digital transmission system in which the abovementioned interferences are reduced, inter alia, as a result of the fact that the received optical signal is divided up by means of an optical splitter into two optical signals for a channel for data extraction and a channel for other functions, in particular clock recovery, respectively. The conversion into electrical signals by means of a photodiode then takes place separately in each channel. A far lower mutual electromagnetic interference of the said channels is thereby achieved. A remaining problem is, however, presented by different signal delay times in the two channels that are due, for example, to temperature differences or to ageing, in particular, of the photodiodes. As a result, a phase shift of the clock signal occurs with respect to the data signal. This then results in incorrect digital identifications in the decision circuit and consequently also in a higher bit error rate.
The object of the invention is to modify an optical receiver of the type mentioned in the last section so that an adaptive phase correction of the clock signal is achieved with which the data signal can then be correctly sampled.
According to the invention, this object is achieved by a method of generating a clock signal from an optical input signal that is divided up optically into two channels, one for data extraction to generate an electrical data signal and one for clock extraction to generate an electrical first clock signal and also a receiver having an optical channel for data extraction and an optical channel for clock extraction, both channels each comprising means for converting the optical received signal into an electrical signal.
Further advantageous embodiments of the invention are to be found in the dependent claims and the description.